As for a fiber-reinforced plastics (FRP) molding method having excellent productivity, a so-called RTM molding method is known, which includes placing a substrate laminate composed of a dry reinforcing fiber fabric in a mold, injecting a matrix resin into the mold to impregnate the reinforcing fiber substrate laminate with the matrix resin, curing the resin, and then demolding the resultant molding. For the production of a relatively large molding or a thick molding, as a highly efficient molding method, such a molding method is often employed that includes, first, draping a reinforcing fiber substrate laminate (e.g., multiple reinforcing fiber substrates) into a predetermined shape to produce a reinforcing fiber substrate laminate preform, which is a molding precursor of an FRP, placing the preform in a mold, injecting a matrix resin into the mold, and then curing the resin with which the substrate is impregnated.
In the production of a preform to be used in the RTM molding, for example, a sequence of steps as mentioned below have conventionally been carried out: (1) a step of placing a substrate laminate produced by laminating multiple substrates on each other in a shaping mold, and then closing the shaping mold to impart a predetermined shape to the substrates by the shaping mold; (2) a step of heating (or preheating) the shaping mold to heat the substrate laminate indirectly, thereby softening or melting a fixing material interposed between the substrates; (3) a step of cooling a preform while keeping the shape of the preform by the shaping mold to solidify the fixing material, thereby fixing the substrate layers to each other; and (4) a step of removing the shaped preform from the shaping mold.
In the sequence of steps, a method of heating using a thermal medium, an electric heater or the like is employed as the method of heating the shaping mold in step (2), and a method of cooling using (ambient temperature or cooled) air, cooling water or the like is employed as the method of cooling in step (3) (see, for example, Japanese Patent Laid-open Publication No. 2011-168009).
In the above-mentioned method in which heating the whole of a shaping mold (generally a metallic mold) using a thermal medium and cooling the whole of the shaping mold are repeated. However, there is a problem in that the time required for one cycle including the heating and cooling is long and the amount of energy consumption required for the heating is large. Therefore, it is difficult to produce a preform having a predetermined shape in a large quantity within a short period of time.
Then, a method is attempted in which an electric current is applied to a substrate laminate or a reinforcing fiber molding in an in-plane direction or a thickness direction and Joule heat generated by electric resistance of reinforcing fibers is utilized to generate heat (see, for example, Japanese Patent Laid-open Publication No. 59-2815 and Japanese Patent Laid-open Publication No. 2009-73132).
That heating method is called a conduction heating method. When the method is used for the production of a preform, only the substrates can be heated without heating the whole of the mold, unlike conventional techniques. That is, heat can be generated in laminated substrates by placing the laminated substrates in a shaping mold composed of an upper mold and a lower mold, closing the shaping mold, pressing the shaping mold, applying an electric current to the substrates in the direction of lamination utilizing the upper and lower molds as electrodes, and utilizing electric resistance of a material constituting each of the substrates to generate the heat. The heat generated in the substrates propagates to a fixing material that is interposed between the substrates and can soften or melt the fixing material. When application of the electric current is completed, the fixing material is cooled by air to atmospheric temperature and solidified and, thereby, the substrate layers are fixed to each other. In this manner, the time required to carry out steps (2) and (3) in the above-mentioned preform production can be greatly shortened. In addition, since the temperature of the substrates can be increased by applying an electric current for a short period of time and the whole of the mold cannot be heated, the amount of energy consumption required for the heating can also be reduced greatly.
However, in the conventional conduction heating methods, a portion to be fixed in a substrate laminate cannot be fixed properly, and a fixed portion cannot be predicted or controlled. Furthermore, there is also a problem that a part of the shaping mold is melted or locally damaged. Therefore, in the conventional conduction heating methods, it is difficult to produce a preform having a predetermined shape with high accuracy and good productivity.
It could therefore be helpful to provide for production of a preform having a predetermined shape, which is a molding precursor of an FRP, employing a conduction heating method, a portion to be fixed in a substrate laminate is fixed properly, the occurrence of the damage of a shaping mold after draping is prevented, the cycle time is shortened to reduce the amount of energy consumption, and productivity of the preform is increased.